To conduct what commonly is known as a drill stem test, a packer and a test valve are lowered into a well on a pipe string and the packer is set to isolate the formation interval to be tested from the hydrostatic pressure of fluids thereabove. The test valve is then opened and closed to alternately flow and shut in the formations while pressure recorders make a record of the pressures as a function of time. From the pressure record, many useful formation parameters or characteristics can be determined. Usually, a sample of the produced formation fluids is recovered.
For drill stem testing in the offshore environment, a test system has been developed which includes a test valve element that is moved between its open and closed positions in response to the application and release of pressure to the well annulus between the pipe string and the well bore wall. This system includes a housing having a spring biased valve actuator mandrel slidably disposed therein, and having oppositely facing pressure surfaces that are arranged such that one face is selectively subjected to the pressure of fluids externally of the housing through the medium of a compressible fluid such as nitrogen gas, while the other face is always subjected to the pressure of fluids externally of the housing body. Thus, during lowering of the tools into a fluid filled well bore, the valve actuator mandrel is balanced with respect to hydrostatic, and the pressure of the compressible medium rather precisely reflects the value of hydrostatic pressure externally of the housing.
The compressible medium is contained in a chamber within the housing, and the hydrostatic head is applied to the chamber by way of a pressure channel that is adapted to be closed at test depth and prior to initiation of the test. Upon closing of the pressure channel the hydrostatic pressure value is "memorized" in the chamber. With the chamber closed, the application of pressure to the fluid in the annulus will develop a pressure differential across the oppositely facing pressure surfaces of the valve mandrel to force it against the bias of a return spring in one longitudinal direction, and a release of the applied pressure will enable the valve mandrel to return in the other longitudinal direction. The foregoing test valve system is disclosed and claimed in the Nutter U.S. Pat. No. Re. 29,638, assigned to the assignee of the present invention, and provides a pressure responsive valve actuator system that is particularly advantageous for use in offshore drill stem testing because the pressure that is applied to the well annulus to operate the valve actuator is a substantially fixed value for any depth in the well that a test may be conducted.
Although the valve actuator system described in the Nutter patent has been widely used, when the system is used to actuate a full-bore test valve element such as a ball valve, difficulty can be encountered in fully reclosing the ball element after it has been opened. This is because the closing force that is applied to shift the actuator mandrel in the closing direction is generated primarily by a coil spring which delivers decreasing pressure as it extends from its compressed to its extended or relaxed length. The pressure that the spring can deliver to effect a fully closed position of the ball valve can be inadequate, particularly under conditions of high formation fluid flow rates. Other factors that inhibit closing of a ball valve include relatively high friction forces that are present due to sliding cam and seal surfaces, which forces may be increased due to presence of debris in the well fluids.
It is an object of the present invention to provide a new and improved pressure controlled tester valve of the type described that can be closed reliably under relatively high flow rate and friction conditions.
Another object of the present invention is to provide a new and improved pressure control tester valve that utilizes the hydrostatic head of the well fluids to assist in closing a balltype closure valve.